What strategy i use depends on what type of map im on, (small or large), and what empire i am (rebel or empire)

Rebellion

Intro:
The rebellion is definately the strongest empire in the game, getting t4-b tanks out is almost always a win if you're a decent player.
T4-b tanks are superior to At-Ats, if you have about 3 squads of t4-bs = 3 built, you will be able to destroy any kind of vehicles that the empire attacks you with, as for infantry, you can just run them over T4-bs are not all that the rebellion has, snowspeeders are probably my favorite unit, with 1 squadron of them early in the game you can take out all of your enemies mining facilities and destroy some of his structures while also forcing him to build anti air turrets and anti air units which will make it much easier for you to defeat his army and attack his base. Rebel Artillery is also far superior to the Imperial Artillery. Rebel artillery has a much large blast radius than the spma-t imperial artillery and on top of that you cant build imperial artillery until tech 3!

Rebels, Small and large map

Rebels are not quite as good on a small map as they are on a large map because the empire likes to rush and it is a whole lot easier when you dont have to move at-sts across a huge map. In every game i play i always build 2 infantry platoons first, this allows me to split up my infantry, if my opponent sends all 4 of his stormtrooper squads to a command point i will not be able to kill him with the 2 infantry squads that i always send, at the beginning of the game split your infantry up and send 2 of them to your nearest mining facility and resource map while sending the other 2 to a command point in the center of the map. Taking the command point in the center is extremely important, once you take it quickly drop your 2 infantry platoons there. If the empire controls the center of the map early in the game you might not be able to stop the rush as instead of having to place at-sts in their base they will be able to place them near yours. On top of that you wont be able to use the mining facilities in the center of the map which are very important if you want t4-bs early. Right after you produce those 2 infantry platoons in the begining of the game begin research on tech 2, you will still have enough credits for a mining facility and resource pad. When you get tech 2 build an artillery and send it to your point in the middle of the map, make sure your infantry are a safe distance ahead of them so that your artillery will be harder to get to by enemy anti-vehicle. Deploy your 2 artillery on your command point and begin building a squadron of snowspeeders, notice the 2 droids that come with your artillery, these can actually be usefull. Select one and use its ability on the enemy starting command point, you will probably see their whole army. If you see 8 or more at-sts you will need to build another artillery and possibly more infantry after your snowspeeders. When your speeders are done send them to an enemy mining facility/resource pad and destroy it, do this with all resource pads they control. Be sure to send your infantry to take them when they are destroyed, you need to have good credit production to get tech 4 and upgrade your t4-bs. If the game is calm begin research on tech 4 and continue attacking your enemy with snowspeeders. If he has no mining facilities go for his power generator or radar. watch your snowspeeders if you're attacking his base, you have to destroy the aa turrets that he builds before they are complete, it is quite easy to destroy a building in production and if you are able to destroy them he will have wasted 800 or more credits which he really cant afford to lose as he shouldnt have mining outposts at this point. as soon as you get tech 4 spam t4-b tanks and upgrade their armor, then weapons, then speed. go for his base with rockets and target his heavy factory first, then light factory, then barracks, ignore the constant spam of infantry that he is probably sending to his command point, if you pay attention to that you will never be able to destroy his base.

Empire

The empire definately arnt as good as the rebellion, but they arnt horrible. you get mobile anti air which can take out enemy bombing runs, AT-AT walkers which are good however not quite as good as a t4-b. The best thing about the empire is your hero units, use the emperor and turn his own t4-bs against him with the emperors convert power, mara jade is also very good early in the game, you can take control of the rebel infantry and place a charge on any tanks or artillery that he controls, severely damaging everything in the area. You also get veers in his AT-AT which cant be destroyed by tow cables and has a very powerful special attack. The empire can be powerful if you use your heroes correctly, they also have a good rush. Also, late game as the empire, watch out for chewie and han. Han will emp your at-ats while chewie captures one, then chewie will fire at your vehicles while they are still unable to move or fire.

Empire: Small Map

build 2 infantry platoons, take 1 resource pad and a command point near the center of the map. Build 3 At-sts and land them at the command point near your enemy. send 1 infantry into his base, check for turrets. keep your army close but out of sight, if he has anti infantry turrets send your walkers first, if he has av send your infantry, take out the turrets and send the rest of your army in. against t2-bs or large squads of infantry use your at-st special attack, this is devastating against t2-bs aswell as infantry. if you can get the shield down take out his light vehicle factory then barracks. once those 2 are down go for his fortress, you dont want him to get tech 2. if your rush fails quickly grab as many resource pads and mining facilties that you can, you will be dead with only 1. The empire is expensive once you reach tech 4, the emperor is 6000 credits and he is key to beating t4-b tanks.

Empire: Large Map

Build 2 infantry platoons, take a resource pad and mining facility and a command point in the middle of the map. research tech 2 after you get your 2 infantry platoons. You will have enough credits for the mining, troopers, and tech. Once you reach tech 2 build a mauler squad unless your enemy is building t2-bs in which case build repulsor tanks. When your mauler is ready start attacking enemy mining facilities and resource pads, if you see infantry do not shoot them, run them over. Maulers dont have much health so they cant take fire from a few platoons of rebel infantry. After your mauler squadron was completed build 2 squadrons of repulsors. Send them into the enemy base, take out the shield generator then attack his fortress, use your special attack on the fortress. 8 repulsors using that special can take it out in about 10 seconds. He will now be stuck in tech 1 if he even got past it, that means no speeders, no artillery, no plex. Build more maulers and repulsors and hit his base.

General Tips:

If you are destroying enemy mining facilties be sure to take them for yourself, making your enemy lose credits is good, making him lose credits and you gain them is better.

Make sure you check your base every few mins, enemies can send units into your base and destroy buildings before you even know they are there.

Dont be afraid to counter attack, this is one of the most important things i can say. Dont let an enemy keep hitting you with armies, you cant let him keep building up. Build turrets and artillery to defend your base then send t4-bs, repulsors, maulers etc. into his base.
This can turn a loss into a win.

Be aggresive about mining facilities and resource pads, take what you can and destroy what you can. Dont let your enemy push you around and take your points.

I agree about the the empire hero's they are key to beating the rebels late game, the emperor with vader is a leathel combination and killing them is a real pain they can take out pretty much anything you throw at them (not speeders but these are easily dealt with), the only thing they can't really do is assault a base well not one with a lot of infantry turrents.

Tip if you've got a good mixed army and you see vader and the emperor run away.